A friend bought a mtb from bikesdirect.com a few years back. It was of similar quality to a lbs bike that might've sold for $500, but i think she got it for $350. All the components were as described on the site. I remember thinking the wheels were really sketchy, with uneven spoke tension and very chintzy rims.... just like a low-end bike shop bike lol. It also had first-generation Tektro Io disc brakes-- which were utterly wretched. But, again,in those days, most lbs mtbs at a similar/slightly higher price point had those same gruesome Tektro Ios (which improved over the years, btw.) It was a very good deal for her, but she had me to put it together. If she didn't, she'd have had to go to a shop for assembly....
During my days at the lbs, ppl would occasionally come in needing help assembling their bikesdirect rigs. The owner really resented bikesdirect and its customers back then, so he'd usually inflate the assembly prices and didn't care one bit about it if they walked out without buying the service. I've known other lbs ppl who felt similarly about bikesdirect customers, but i tend to see every customer as an opportunity. An opportunity to make a fair profit on the assembly, and an opportunity to build a relationship with the customer so s/he'll (hopefully) buy the next bike at the lbs.... The guy i used to work for has definitely mellowed out as far as online shoppers needing service goes....
Short answer: if you're competent with assembly, buy without fear but make sure you go over the wheels. IF you're not a good wrench, be prepared to pay a shop to put the bike together for you. The bike will arrive mostly assembled, much like the lbs receives their new bikes, but there's still a good bit to bolt together, and a lot of tuning. Just to reiterate: my biggest concern would be the wheels---not so much the components used to build the wheels, but the wheel build itself will likely be incomplete....